1. Field of the Invention
This invention relates to an X-Y positioning system and in particular, to an ultra precise and fast electronic positioning system for use with electron beam lithography.
2. Prior Art
Electron beam lithography systems are employed as tools for the formation of integrated circuit devices. Exceptional precision and high throughput are desired characteristics of E-beam lithography systems. A standing requirement in this technology is to define an apparatus which can be used to position the target, that is, the semiconductor wafer or glass mask under the E-beam for processing. Such a positioning system must offer exceptional accuracy, compatible with that of the E-beam system itself, yet have adequate speed during the positioning sequence and operations to not hinder the overall throughput in the system.
Typically, such systems are used for orthogonal, X-Y positioning and are used to move a table carrying the wafer within a specified range, for example, 200 mm, yet within that range establish a relative position of the table, vis-a-vis the electron beam to within exceptionally high tolerances. Contemporary positioning tolerances are within the range of .+-.0.04.times.10.sup.-3 mm. Moreover, the positioning system must be capable of moving from one location to another in the shortest possible time to improve the total throughput of the E-beam lithography tool. Typically, move times for a 5 mm reposition in the order of 125 ms have been achieved within the prior art servo systems and have been employed in positioning circuits using active position information feedback.
There is, however, a fundamental trade-off when conventional prior art servo systems are employed between absolute positioning accuracy and speed of movement. The trade-off is conventionally expressed as one of energy management, that is, a first criteria of supplying enough energy to the system to move it quickly from one position to another and to then supply sufficient energy to the system as the X-Y stage is being braked to a stop. The stopping of the table with the minimum amount of residual oscillation or "ringing" requires that an exact amount of energy be applied to the stage at all times. Accordingly, the achievement of fast move times requires that the table be stopped with the minimum of ringing.
The problem of energy management in the fundamental trade-off is compounded by considering the relative dimensions which are subjected to this energy application. For example, within the dimensional range of 0.04 .mu.m, structural members which can normally be considered as rigid, become non-rigid. Hence, to prevent undue stresses on these members, and yet quickly dampen out undesirable effects (ringing), requires that energy applied during the stopping be applied in a measured manner in order to more quickly establish the absolute true position of the table.
A prior art system using a feedback servo loop is shown in FIG. 1. Such a conventional positioning loop servo drives to a null condition when the input to the digital-to-analog converter (DAC) reaches zero. This system generally employs a computer controller 10 which provides position information concerning the ultimate destination, that is, the desired location of the table, vis-a-vis that of the E-beam. The destination information is fed to a subtractor 11 which also receives actual position data from a laser transducer 14. The difference signal represents the difference between destination position and actual position. The difference signal is fed to the digital-to-analog converter (DAC) 16 which is employed as an input to a conventional servo speed loop motor drive comprising amplifier 18, motor 20 and tachometer 22. In accordance with such conventional systems, the motor rotational speed information is sensed by tachometer 22 which provides a speed signal to the comparator amplifier 18 used to drive the motor 20 and hence, the X-Y table 24. As is apparent, when the input signal to the DAC 16 is zero, the actual position equals the destination position and the system has been driven into a null such that the motor is stopped.
Prior art X-Y positioning servo loops, as shown in FIG. 1, have a number of significant disadvantages. First, such servo loop systems tend to dither, that is, oscillate about the null point. Such oscillation results in a deterioration in throughput since the E-beam registration and/or write operation cannot occur until such oscillations have been effectively damped. Moreover, the energy inputs to such systems are generally constant and as a result, the X-Y stage tends to move at a constant speed until it reaches the null position where at that point braking occurs. This constant application of energy to the table is a further contributing factor to "ringing". Importantly, such prior art systems tend to operate within a rather wide deadband which is generally defined by constant signal inputs into the servo loop.
Such a technique of compensation for minimizing deadband are known in the prior art as shown, for example, in the U.K. Pat. No. 1,080,108. In the system of this prior art patent, digital compensation for the deadband in a servo mechanism is used but, the amount of compensation is predetermined. Specifically, a pair of counters are employed which cycle in synchronism at the same frequency such that the difference in the contents of one of the counters with respect to the contents of the other causes movement of the servo system. As a result, oscillation around the null point tends to occur when deadband compensation, such as shown in FIG. 2 of the reference is employed.
U.S. Pat. No. 3,701,992 relates to a device for minimizing the servo loop deadband in a recorder. It suffers from the same defects as the previous reference. In this patent, a second servo amplifier is placed in parallel with the main servo amplifier comparable to element 18 in FIG. 1 of the present application. The first amplifier produces an output proportional to the analog pin control input signal. A predetermined constant output signal is supplied by the second amplifier to overcome the deadband caused by friction encountered in the mechanical element, in that case, a graphical recorder. The outputs are summed to drive the servo motor. However, even in the '992 patent with the use of a second amplifier 43, coupled in parallel with the first amplifier 35, oscillations around the null point tend to occur. This is because, as in the case of the U.K. patent, a constant signal is used to reduce deadband. Since the deadband constants are recognized as not being the same over the totality of the range of movement, the deadband can never be completely eliminated when a constant signal is employed. That is, it is recognized that the deadband constants vary from one position to another in a X-Y stage and consequently the use of a constant signal will not completely eliminate the deadband.
U.S. Pat. No. 3,821,625 also relates to deadband compensation in a servo mechanism by employing a second amplifier with a predetermined fixed output voltage. This output, in the form of an incrementally small basic control signal causes sufficient energization to be applied to the device to initiate immediate movement of the driver element. The reference is therefore fundamentally no different from the remainder of the prior art in its application of a constant signal to reduce the deadband.